Patent GB-659814-A (GB)
Title: Improvements in and relating to receivers for angular velocity modulated waves
IPC: H03D11/06
Filed: 1947-09-01
Abstract: 659,814. Frequency modulation; valve generating circuits. PHILCO CORPORATION. Sept. 1, 1947, No. 24098. Convention date, Sept. 13, 1946. [Classes 40 (v) and 40 (vi)] In a receiver of angular - velocity modulated waves, the incoming modulated carrier is heterodyned by a local oscillator whose frequency is commutated by supersonic pulses. The resulting intermediate frequencies IF1, IF2, Fig. 2, are applied to a discriminator circuit resonant at a frequency fo midway between them, and the discrimmator outputs are channelled to separate detectors by pulses synchronous with those producing the commutation of the local oscillator frequency. The signal is recovered as the difference of the two detected outputs. The discriminator circuit may be in backcoupling relation with a super-regenerative valve amplifier which is quenched and unquenched by the commutating pulses. The local oscillator comprises a multi-vibrator pair 2, 3 having the normal reciprocal connections for producing recurrent pulses. A tuned circuit 11, 12 is connected between the grids and earth and a tapping on the coil 11 is connected directly to one cathode and through a coil 13 to the other, so that different frequencies are produced during the respective conductive periods of the valves. These frequencies are mixed with the incoming FM wave in a valve 20 and produce commutated intermediate frequencies IF1, IF2 in the output network 21 ... 24. The coil 24 is coupled to the discriminator circuit 25 tuned to the midfrequency fo, Fig. 2. As shown, the circuit 25 is back-coupled to a super-regenerative valve amplifier 28 and is shunted by a valve 39 which quenches the circuit 25 when. it is rendered conductive by positive pulses oil its grid. In Fig. 4, the curves A, B represent the voltage on the grids of the valves 1, 2, and C shows the voltage at the junction of two resistors 34, 35 connected between the grids. The voltage C is applied by lead 38 to the grid of the quenching valve 39, and is such as to quench the circuit 25 towards the end of each pulse period, and to unquench it immediately afterwards. Curve E, Fig. 4, shows in curves 41, 43 the rise in oscillatory voltage in the circuit 25 for the condition of Fig. 2 in which each IF is equally displaced from the resonant frequency fo. When, however, the input frequency is displaced by modulation, the condition of Fig. 3 arises in which one IF approaches and the other recedes from the frequency fo. In this case, the oscillatory voltage in the circuit 25 rises along the curves 45, 47 as shown at G, Fig. 4. Voltages corresponding to the curves E, G are present on the anode of the valve 28 and are applied to the grids of two valves 51, 52 which are rendered operative in turn by positive pulses D, Fig. 4, applied to their anodes and derived through resistor-condenser combinations 57, 58 and plate resistors 55, 56 from the respective anodes of the multivibrator valves 1, 2. In this way the pulses including the curves 41 or 45 are passed through the amplifier 51 to the grid of output valve 69, appearing there in the form shown at F and H, whilst the pulses 43, 47 appear on the grid of output valve 70. The detected signal is taken across the anodes of the valves 69, 70 and represents the difference of the voltages provided by the respective channels. For the condition E, F no output is obtained, whilst for the modulated condition G, H an output is obtained proportional to the instantaneous frequency deviation of the incoming wave. The pulse or quench frequency is removed by the provision of condensers 90, 91, between the grids of valves 69, 70 and earth. The parallel curves shown in E ... H, Fig. 4, represent the effect of changes of amplitude of the incoming wave, and cancel out in the differential output. Specification 659,658, [Group XXXV], is referred to.